Patent GB-588356-A (GB)
Title: Improvements in or relating to electrical signalling systems
IPC: B61L1/20
Filed: 1945-02-14
Abstract: 588,356. Electric signalling systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd., and THOMPSON, G. W. Feb. 14, 1945, No. 3646. Addition to 546,304. [Class 40 (i)] The system described in the parent Specification for the identification of a station calling on a single pair of conductors by the automatic measurement of the resistance of the line is so modified that the resistance of the automatically adjustable element of the Wheatstone bridge measuring device is normally intermediate its maximum and minimum values. -Three equal arms of the bridge, YA, YB and VR, Fig. 1, remain fixed. The fourth arm includes the line 10, 11, Fig. 2, are normally only four .units YC ... YF of the nine resistance units YC ... YL forming the adjustable element. Closure of a key SK on the line to give a stop signal to the engine operator operates relay L which is followed by LL to complete the bridge circuit, to ring a bell over leads 12, 13 and to introduce relays AX ... BZ, Fig. 3. Two polarized test relays BRA, BRB in the diagonal operate according to the direction of the test current to adjust the resistance of the adjustable element by operating, as necessary, first AX or BX for r3 unit adjustments, followed by AY or BY for r2 units and finally by AZ or BZ for r1 unit. The adjustment ceases when neither test relay operates. The same relay operations prepare a circuit for the appropriate indicator lamp to be lit when h6; Fig. 4, is closed. They also store the signal-by releasing selected relays of a group of normally operated magnetic locking relays MAX ... MBZ which' copy the original lamp circuit connections on a duplicate series of contacts max ... mbz. When the.bridge is balanced, relay H is operated through contacts bra, brb. Relays, FK and FJ follow and the appropriate indicator lamp is lit steadily by positive over KCS. The operator stops the engine and operates the first cancellation key KCA which releases Hand operates PH which locks on fh1. Relays FJ and FK now interact to apply a flashing signal over contacts fj3, fk3 and the duplicate lamp contacts controlled by the storing relays. A new start signal from the same point operates H again, and gives a steady indicating light over fh4 and h6, the flashing signal being meshed. A start signal from another point would rebalance the bridge, give a steady light on-the corresponding indicator lamp but would leave the first lamp flashing, FH and the storage relay remaining unchanged. On receipt of the start signal from the correct station the operator presses cancellation key KCB to return the equipment to normal. By connecting terminals 14 and 16, Fig. 3, instead of 14 and 15, it can be arranged that on operation of the cancellation key KCA, the indicator lamp is extinguished until the receipt of a further call when it flashes if the two calls are not from the same station. A key KT facilitates adjustment when the system is first installed by cutting out the storing relays and the flashing device and so permitting the receipt of signals from various points in quick succession without operation of the cancellation keys. If the line develops a fault, key. KCW is operated to cut-off the line and CW follows to light a warning lamp CWL. The position at which the fault occurred may then be determined by operating key KCS which lights the corresponding lamp over the contacts of the magnetic locking relays. Leads 19 and 20, Fig. 2, are provided for the connection of a telephone to the line. Leads 17 and 18 are test leads. Specification 515,529, [Croup XXXVII], is referred to.